The process for preparing dough into sheets such as for ethnic Mexican foods including tortillas, corn chips and the like requires a masa to be made from wet milled corn or corn flour, water and other additives. For certain other products, the dough may be made from rice, wheat flour and bean flour, etc. The dough or masa is processed into sheet form by being compressed between pinch rolls and cutters are then applied against the front roll (the roll nearest to the take out conveyor) to develop the selected shape for the product. After cutting, the product may be further formed and cooked as by toasting, baking or frying, or in the case of tortillas, the product may be heat treated and packaged for later use.
The sheeting operation can be of critical importance in preparation of food products of this general type because of the natural variations of the physical properties of the dough or masa. It has been observed that the visco-elastic mass properties of the dough may vary as often as every 20 minutes which is a typical batch duration. The variations in coarseness or particle size of the corn flour, adhesive properties and moisture content in the masa influence the quality of the sheeted product. Many of the prior art sheeters were incapable of accommodating the wide variations in the viscoelastic mass properties of the masa dough and were unable to produce a product within specifications. The operators of such equipment would frequently completely discard entire batches of masa because the machines could not maintain product specifications. This was costly in lost production time and wasteful of raw materials.
Particularly in constructions where the sheeting rolls had stripper wires to separate the formed sheet below the nip, special attention was required for the proper position and tension of the stripper wire to assure adequate force between the stripper wire and the roll. Commonalty, the back roll was machined to a convex profile, an exacting and expensive operation, and the front roll was machined to a complimentary concave prof fie in order to ensure an accurate gap or nip between the rolls. In the case of the concave roll, the position of the stripper wire is skewed at an awkward angle and the products stripped from the roll fall different vertical distances onto the conveyor there below can saligned laterally on the conveyor. The Oct. 30, 1990 U.S. patent to Joseph L. Mistretta, U.S. Pat. No. 4,966,541 addressed the difficulties of positioning the stripper wire against the convex face of the back roll and implied that the existence of the stripper wire was critical to effective and efficient sheeting of masa. The Jan. 19, 1993, U.S. patent to Mistretta, U.S. Pat. No. 5,180,593, shows driving the back and front rolls at the same surface and positions stripper wires on both the front and the rear rolls. The rear roll and cutter roll are driven from the front roll through a spur gear arrangement. There was apparently no perception in the prior art of selectively varying speeds over a wide range of either the cutter or the back roll to keep the products within specifications and to achieve efficient and effective sheeting without the use of a stripper wire or a doctor blade engaging the masa adjacent to the back roll. The elimination of the stripper wire or doctor blade and their attendant problems of adjustment and constant repositioning is an objective long sought.
In the processing of the products, such as masa, it is common to find that the material for dough varies in consistency from batch to batch. The machine operators will find the masa to differ in particle size, stickiness, cohesiveness, abrasiveness, flowability, and similar characteristics. This has been found to influence the shape, weight, thickness and quality of the final product. To accommodate some of these variables, it has been the practice of changing the speed of the machine pinch rolls such as by changing the gearing of the front and back rolls as well as the gearing of the cutter. The limitations of available gear sets is a problem when the operator needs a speed change that lies within less than one gear tooth, that is to say, the calculated fraction of a gear tooth. One producer has changed the speed of the back roll to rotate slower than the front roll and has eliminated use of the stripper wire or doctor blade. Although changing gearing helps to accommodate variations in the product characteristics, it is recognized that it is costly not only from the standpoint of the mechanics labor in effecting the change, but also in the down time of the production line in which the sheeter operates. Frequently the masa during down time will go "off" flavor and must be discarded.
As the material properties change, the size of the die cut product will change in the direction of travel at a varying rate as it is striped from the front roll and the size and weight of the finished product is directly affected by the varying masa properties and nay drift outside of the allowed specifications. Thus, it is highly desirable to accommodate the variations in the masa while making the necessary change to the sheeter without shutting down the entire processing line.